CRASH syndrome in humans, which includes X-linked hydrocephalus, is caused by mutations in the gene encoding the Ll cell adhesion molecule. There are no good experimental models in which brain syndromes, caused by various mutations in Ll, can be studied easily. The long-term goal of this exploratory project is to create a new experimental model of anomalous brain development that can be used to study in vivo the cellular effects of different mutations in L1. The overlying hypothesis to be tested is that cell behavior can be altered in the developing chicken embryo brain by ectopic expression of specific mutated Ll cDNAs, or by attenuation of endogenous Ll using direct gene conversion or antisense sequences. Human Ll mutations can be grouped according to the type of mutation. Most, but not all, severe hydrocephalus, grave mental retardation, and early deaths are caused by mutations that produce truncations in the extracellular domain (Class 3 mutations), which probably are secreted and interfere with other adhesion and signaling systems. It is theorized that similar mutations in the chicken homologue of L1, called NgCAM, will interfere with neuronal behavior (e.g., neuronal migration) when ectopically expressed in the embryonic chick brain. We also theorize that a malformed brain will result when such mutations are expressed throughout the tissue, but that localized expression will inhibit neuronal migration and axon extension only in the immediate area. Specific Aim 1: Define the developmental effects of widespread NgCAM misexpression. Hypothesis 1: A generally malformed brain will be generated in the developing chick by widespread introduction of antisense-NgCAM cDNAs, Class 3 mutant NgCAM cDNAs, or of gene conversion DNA oligonucleotides by in ovo electroporation. Specific Aim 2: Define the developmental effects of isolated NgCAM misexpression. Hypothesis2: Introduction of mutated and antisense NgCAM cDNAs in discrete, marked cell clones by retroviral vectors will inhibit neuronal migration and axon extension in some, but not all, neurons expressing such sequences. Specific Aim 3: Define the effects and distribution of mammalian Class 3 Ll misexpression. Hypothesis 3: Class 3 Ll mutations can operate beyond the cells that express them.